1. Field of the Invention
The present invention relates to a deterioration determination device for a catalyst, a catalyst deterioration-determining method, and an engine control unit, for determining deterioration of a catalyst for reducing exhaust emissions, which is provided in an exhaust passage of an internal combustion engine.
2. Description of the Related Art
Conventionally, there has been proposed a deterioration determination device of this kind in Japanese Laid-Open Patent Publication (Kokai) No. 2001-271696. In this deterioration determination device, by making use of characteristics of a catalyst that when the catalyst is deteriorated, the oxygen storage capability thereof, deterioration of the catalyst is determined as follows: Oxygen concentration is detected on an upstream side and a downstream side of the catalyst by an upstream oxygen sensor and a downstream oxygen sensor, respectively. Further, absolute values of the differences between values detected by the upstream oxygen sensor and values detected by the downstream oxygen sensor are integrated, and if the integrated value is smaller than a predetermined reference value, it is determined that the catalyst is deteriorated. Further, a time required for exhaust gases to reach the downstream oxygen sensor from the upstream oxygen sensor (hereinafter referred to as “the delay time”) is calculated based on an intake air amount indicative of a flow velocity of exhaust gases, and in association with each value detected by the downstream oxygen sensor, each value detected by the upstream oxygen sensor before the delay time elapses is used for the calculation of the above-mentioned difference between the detected values by the respective sensors. Thus, in the conventional deterioration determination device, the values detected from substantially the same exhaust gases by the upstream oxygen sensor and the downstream oxygen sensor are used for the deterioration determination.
However, in the above-described conventional deterioration determination device, merely the above-described delay time of the exhaust gasses is added to the deterioration determination of the catalyst, and hence there is a fear that it is impossible to accurately determine deterioration of the catalyst due to influence of variation in output characteristics caused by differences between individual products of the upstream and downstream oxygen sensors, variation in exposure to exhaust gases caused by different mounting conditions of the sensors, response delays caused by aging of both the sensors, and so forth. Particularly, since the integrated value of the differences between the values detected by the upstream oxygen sensor and the values detected by the downstream oxygen sensor is used for the deterioration determination, even if the deviation of an output value from a true value caused by variation in output characteristics of the both sensors is relatively small, this greatly influences the calculated integrated value. In this case, the accuracy of the deterioration determination is much lowered.
To prevent such a problem, for example, it is envisaged to compensate for the influence of variation in output characteristics of the upstream and downstream oxygen sensors on the deterioration determination of catalyst. However, in this case, if the degree of aging of the upstream and downstream oxygen sensors is high enough to definitely determine the aging, it is possible to properly compensate for the influence according to the result of the determination. However, if the degree of aging of the both sensors is low, the deviation from the true value is also small, and hence it is impossible to properly compensate for the influence of variation in the output characteristics on deterioration determination of the catalyst. This makes it impossible to accurately determine deterioration of the catalyst. This fact is similarly applied to the influence of variation in output characteristics due to differences between the individual products of the upstream and downstream oxygen sensors, different mounting conditions of the sensors, on the deterioration determination of the catalyst.